GENERA OF ALG^E. 31 



D. In a sheath or a jelly mass (K). 



D. Not sheathed nor in jelly (P). 



E. Branched at almost right angles to the stem; cells in a single se- 



ries Hapalosiphon, 92 



E. Branched irregularly or with twin branches (F). 



F. Cells in single series in young plants, several series when older, 



Sirosiphon, 91 



F. Cells in a single series only (G). 



G. Sheath broad, striate, hyaline, wing-like . . . Petalonema, 90 

 G. Sheath not wing-like (H). 



H. Sheaths distinct, not agglutinated to one another (I). 



H. Sheaths agglutinated in erect, wick-like bundles, 



Symphyosiphon, 87 



I. Heterocysts present (J). 



I. " not present; sheath with a single bluish filament, 



Plectonema, 89 



J. Heterocysts disconnected from the twin branches, Scytonema, 86 



J. Heterocysts near the usually single branches . . Tolypothrix, 88 



K. In a sheath; filaments moniliform, curved .... Nostoc, 93 



K. In a sheath; filaments not moniliform (Q). 



K. In mucilage or jelly mass (L). 



L. Jelly mass spherical; filaments transversely plicate, radiate, 



Glceotrichia 84 



L. Jelly mass hemispherical, filaments radiate . . . Rivularia, 85 



L. Jelly mass a stratum or indefinite (M). 



M. Filaments erect, attached at base, jelly stratum flat, often 

 incrusted Isactis, 83 



M. Filaments not attached at base, not moniliform, heterocysts ter- 

 minal, single Cylindrospermum, 98 



M. Filaments not attached, not moniliform, colorless, heterocysts 

 none Crenothrix^ 99 



M. Filaments moniliform, cells globose or elliptical (N). 



M. " cells compressed disciform (O). 



N. Heterocysts intercalated in the filaments, 



Anabcena, 94, or Sphczrozyga, 95 



O. Heterocysts intercalated at regular intervals, yellow, 



Nodularia, 97 



P. Bluish, bluish-green, blackish (T). 



P. Whitish or silvery; in sulphur waters Beggiatoa, 105 



Q. Two or more filaments in each sheath . . . Microcoleus, 103 



Q. One filament only in each sheath (R). 



